Two former Parramatta Eels players are accused of harbouring semi-automatic weapons and possessing more than half-a-million dollars in cash after dramatic arrests in Sydney's Centennial Park yesterday.

A Langwarrin man says he is outraged after he received an anonymous letter threatening to kill his Samoyed dog by “throwing large amounts of poison over the fence”.

'Tony' said the terrifying letter was unprovoked and he holds concerns over his daughter's safety as the letter also made reference to the five-year-old, who sometimes plays in the courtyard with the family’s dog, Marley.

“When my partner read the letter to me over the phone, I just couldn’t believe that someone would write about my daughter playing on the street and threatening to kill my dog,” he told 9NEWS.

“It’s definitely frightened us.”

He doesn’t believe the letter, received earlier this month, came from immediate neighbours, but rather someone driving by who took offence to the dog playing in the street.

“They’re trying to make us feel some sort of pain… to write something like this two weeks out from Christmas just makes you think why someone would try and scare a whole family,” he said.